GtkListBox: Remove add_to_scrolled()

GtkListBox: Remove add_to_scrolled()

Now that we automatically set up the focus_vadjustment we only
need the adjustment to handle the page-up/down scroll length correctly.

We use parent_set to automatically pick up the adjustment from a
scrollable (i.e. viewport) parent which makes add_to_scrolled useless, so
it can be removed.